This invention relates to a process for removing sulfur oxides from a waste gas stream and recovering elemental sulfur. More specifically, this invention relates to a catalytic fused salt extraction process for removing sulfur compounds, particularly sulfur dioxide and sulfur trioxide, from waste stack gases, regenerating sulfur trioxide gas from the molten salt and reacting the sulfur trioxide with hydrogen sulfide in a Claus reaction to produce elemental sulfur.
Significant quantities of sulfur oxides and especially sulfur dioxide are present in the waste stack gases of many industrial installations including metal refineries, chemical plants, pulp mills, oil refineries and fossil fuel burning power generating plants. Despite extensive research efforts to reduce environmentally hazardous sulfur oxide discharges, few practical methods for the cleansing of waste stack gases have been proposed.
Methods of gas purification which are the most attractive are those which can be implemented at a relatively low capital cost and which are able not only to cleanse waste stack gases of sulfur oxides but also to transform recovered sulfur oxides into valuable byproducts at reasonably low operating costs. The preferred byproduct is elemental sulfur which can be most easily stored, transported or converted to a variety of useful compounds.
U.S. Pat. No. 3,789,110 to Ball illustrates a process in which sulfur dioxide from waste stack gas in absorbed onto activated carbon as sulfuric acid. The sulfuric acid laden activated carbon is subsequently reacted with a correct proportion of hydrogen sulfide to reduce all the hydrogen sulfide and a portion of the absorbed sulfuric acid to elemental sulfur. Elemental sulfur and the remaining sulfuric acid are then reacted to form concentrated sulfur dioxide without reaction with the activated carbon. The sulfur dioxide is recovered and the completely regenerated activated carbon recycled to react with additional waste stack gases. The recovered sulfur dioxide gas may be further reacted to form byproducts such as hydrogen sulfide, elemental sulfur or sulfuric acid. This and a wide variety of similar systems proposed for cleansing gases of sulfur oxides and converting the recovered sulfur oxides into useful products require a multiplicity of steps which are both cumbersome and costly. In addition, many of the proposed systems employ steps which are not sufficiently efficient at removing sulfur oxides from the waste stack gases to meet current pollution standards without a secondary treatment of the gas streams.
In recent years it has been determined that sulfur oxides may be removed from gas streams with high efficiency by contacting the gas stream with molten salt mixtures under certain conditions. U.S. Pat. No. 3,552,921 to Blytas describes a catalytic fused salt extraction process utilizing essentially a molten salt of potassium sulfate and potassium pyrosulfate to remove sulfur dioxide from waste stack gases. The extraction process forms a pyrosulfate-rich salt solution. To regenerate the molten salt, the pyrosulfate-rich solution is heated to a temperature above 600.degree. C. to reverse the process and form potassium sulfate and sulfur trioxide.
My U.S. Pat. No. 3,855,386 described an improved system of sulfur dioxide removal from gaseous streams which operates at lower temperatures and less corrosive conditions. This system involves contacting the gaseous stream with a molten potassium sulfate salt mixture having dissolved V.sub.2 O.sub.5 and recovering sulfur trioxide from the salt mixture.
The use of a Claus reactor to convert sulfur oxides in waste stack gases into elemental sulfur is mentioned in the EPA Technical Report No. EPA-650/2-74-085-b. This report also describes the acid leaching of neutral roasted copper concentrates for the production of hydrogen sulfide and underscores the necessity of developing a method of scrubbing sulfur oxides from waste stack gases which is of greater efficiency than the unaided Claus reaction.